DE-A-33 30 560 discloses an electric chain hoist. This electric chain hoist contains, in a housing, an induction motor which is provided with a cylindrical armature and, via a slipping clutch, drives a gear input shaft. The output shaft of the gear is coupled in torsionally rigid fashion to the chain sprocket of the electric chain hoist.
At the end of the armature remote from the gear input shaft there is a cone brake which is preloaded into the braking position by a spring. To release the brake, a solenoid plunger is provided. This solenoid plunger is coaxial with the motor armature and is attracted when the motor is set in rotation.
Owing to the arrangement, the brake acts via the slipping clutch and consequently cannot generate a greater braking torque for the chain than the maximum that the slipping clutch will allow. Since static friction is generally greater than sliding friction, a situation may arise in which the hoist is no longer able to hold a load hanging from the hook, even though the brake is engaged, because the clutch is slipping. Such a situation can arise if, when raising the load, the clutch is initially operated in the static friction range but, because of a fault, for example temporary entanglement of the load with an object, or due to longitudinal vibrations in the chain, the torque limit of the slipping clutch is then exceeded and the clutch passes into the state of sliding friction. Even if the armature is then stopped and the holding brake is applied, the load continues to fall since the slipping clutch may no longer be able to return to the state of static friction from the state of sliding friction.
It is therefore expedient to place the brake in that part of the drive line of the hoist which comes after the slipping clutch, taking the motor as the starting point.
One such solution is, for example, known from DE-A-44 08 578, which is not a prior publication. The braking torque can be set independently of the slipping torque of the slipping clutch and, as a result, the hoist is capable of holding loads reliably even when the slipping clutch has already started to slip under limiting-load conditions.
With this solution, however, the time relationship between the starting of the motor and the release of the brake must be maintained with great precision. If, for example, the brake were released more rapidly than the motor can start up while raising a load, the load would initially sink before being raised by the motor. If, conversely, the motor started up first and the brake were released afterwards, the applied brake would cause the slipping clutch to slip and it would then be possible to raise only a relatively small load with the hoist, in accordance with the friction coefficient under conditions of sliding friction.
The solution given in DE-A-37 10 332 avoids this problem by using a sliding rotor motor which ensures synchronicity between the motor and the brake with absolute reliability. However, such sliding rotor motors with a conical armature are very expensive to manufacture.